In a gas turbine engine, oil is distributed to various components, such as bearings, for cooling and lubrication. The oil is heated as it circulates around or through these various components. The oil can be cooled by cooling air or fuel flowing to the combustion chamber. Cooling air is typically taken from the fan, which reduces thrust of the engine. Fuel flowing to the combustion chamber can also be used to cool the hot circulating oil. Rejecting heat from the oil into the fuel incurs few of the penalties of air cooling. However, the amount of rejected heat is limited by the maximum temperature tolerable by the fuel.
The fuel system of a gas turbine engine includes a fuel pump for pressurizing and transporting the fuel through the system to the combustion chamber. The fuel pump is generally a boost stage and single positive displacement main stage which is attached to the gearbox such that the speed of the main fuel pump is proportional to the engine speed. At certain conditions, such as cruise, the engine operates at a relatively high speed while a relatively low fuel flow is required. Further, the main fuel pump stage is typically sized by high power or start conditions, resulting in extra flow capacity at all other engine operation conditions. In this way, the main fuel pump stage results in excess fuel flow. The excess fuel is recycled through a bypass loop to the low pressure side of the main pump. At low fuel requirements, the fuel may be recycled several times before being sent to the combustion chamber. The combination of recycling excess fuel and pump inefficiencies increases the temperature of the fuel. This additional heat limits the amount of heat that can be rejected into the fuel from the circulating oil. Reducing the amount of heat rejected into the fuel by the fuel pump would improve engine performance. Further, a large amount of time spent is in the cruise condition during a flight, and reducing the amount of heat rejected into the fuel by the main pump during the cruise condition may have a larger impact on engine performance than similar reductions during other flight conditions.